If i use Wi-Fi does it effect my Data Use?

I have a iPhone 4 please help

If you use Wi-Fi to get access to the Internet, it will not be using your cellular data plan, and therefore will not affect the Data use.

You just need to be aware whether the phone has actually successfully joined the Wi-Fi network before you start browsing and downloading. You don't want to think you are on Wi-Fi when you are actually on cellular data.